1. Field
The disclosed embodiments relate to a method and an apparatus for correcting the output signal of a radiation sensor and for measuring radiation. Related disclosures can be found in DE 102 004 028 032.0 and DE 102 004 028 022.3.
2. Brief Description
Radiation sensors transform electromagnetic radiation into an electrical signal. This may be accomplished, for example, by thermopiles, bolometers or the like. The radiation sensed by them is often infrared radiation (wavelength larger than 800 nm). Radiation sensors of this type are often used for contactless temperature measurement. The body of which the temperature is to be measured emits radiation in dependence of its temperature. The radiation is the more intense the higher the temperature of said body is. Accordingly, the emitted infrared radiation of a body may be used for contactless measuring its temperature. The details thereof will be explained with reference to FIG. 1.
FIG. 1 shows a sensor element 10. It comprises a frame 2 which is a support for a membrane 3. The frame 2 surrounds an opening 4 which may have rectangular or round cross-section depending on particular necessities. The membrane 3 serves to thermally insulate the actual sensing portion 1 formed on the top surface of the membrane 3 from the surrounding as far as possible. From the top surface, the sensing portion 1 of the sensor element 10 receives radiation, preferably infrared radiation, as indicated by two arrows IRn and IRs. IRs indicates the desired signal radiation from the body to be measured. However, the sensing portion receives also noise radiation, as indicated by arrow IRn. This may come from components in the immediate vicinity of the sensing portion, for example the housing of the sensor, shielding members, or the like. The sensing portion 1 itself cannot distinguish which kind of radiation impinges on its surface. It will transform both of them into an electrical signal.
If the sensing portion 1 comprises a thermopile consisting of a sequence of hot and cold contacts, then the measurement principle is that the incident radiation will transform into a temperature change (usually rise of temperature) at the hot ends/contacts 1a. In FIG. 1, the ends above the opening 4 are the hot ends 1a of the thermopile, whereas the ends above the frame 2 are the cold ends lb. For enhancing measurement sensitivity, the hot and cold ends may be covered with auxiliary layers, particularly an absorbing layer 5 above the hot ends 1a and a reflecting layer 6 above the cold ends lb. The incident radiation causes a difference in temperature between the hot and the cold ends, and in dependence of this temperature difference, the thermopile will generate an electrical signal.
Another noise source is indicated by the thick arrow Ta. It is heat conduction through the various physical bodies. 7 is a substrate such as a silicon wafer, a ceramics baseboard or a printed circuit board on which the sensor element 10 of FIG. 1 is mounted. Changes in the ambient temperature will communicate through heat conduction through the support 7, frame 2, and membrane 3 to the sensing portion 1. Heat conduction also takes place between the surrounding atmosphere and the sensor element 10 and the sensing portion 1 thereof, but heat conduction through the substrate 7 is usually much stronger in effect. Since the cold ends are usually differently located with respect to the frame 2 as the warm ends, the former will experience a change in ambient temperature earlier than the warm ends. The hot contact on the membrane of the sensor element is usually the last relevant component that experiences a temperature change because it is usually the thermally best isolated part of the relevant measurement system.
Thus, a change in ambient temperature will first be experienced by the cold ends and only later by the warm ends of the sensing portion 1. Accordingly, through heat conduction a temperature difference builds up between the hot and the cold ends which has nothing to do with the temperature difference caused by the signal infrared radiation. The temperature difference caused by heat conduction will be the larger the faster the temperature change is, because in a fast transition through a temperature range the sensor element will not go through the temperature range in a state close to thermal equilibrium. It will not have almost the same temperature everywhere on the sensor. Rather, there will be temperature differences between the hot and the cold ends which serve to cause errors in the output signal and accordingly in the measured temperature.
The above two mentioned German patent applications of the same applicant propose various ways for overcoming erroneous measurements caused by temperature shocks of the ambience. One proposal is to equalize the thermal flow towards the hot and the cold ends by arranging them suitably with respect to the frame 2 on the one hand side, and on the other hand side by appropriately designing the auxiliary layers 5 and 6 (absorbing layer, reflecting layer). However, in various applications this cannot fully eliminate erroneous measurement. In many cases, it is desired to have the cold ends above frame 2 because it serves as a thermal mass and has the effect of keeping the cold ends at a steady temperature when measurement is made. Accordingly, there is a systematic desire for an asymmetric arrangement of the hot and cold ends with respect to the frame 2, and the design of the auxiliary layers cannot fully compensate this for changes of the ambient temperature.
Another proposal is to design the housing of the sensor element 10 such that noise radiation as symbolized by arrow IRn is blocked from the sensing portion as far as possible.
But while the above proposals have significant advantageous effects particularly by appropriately designing the components that are needed anyway (sensor element 10 including frame, membrane, thermopile, auxiliary layers, and also the housing of the sensor), there are nevertheless situations where an even more sophisticated compensation of error sources particularly at changing ambient temperature (“thermal shock”) is desired.